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Danny has a tendency to follow his intuition, as opposed to the evidence itself; a fact that has caused him to butt heads with Mac on several occasions. (episode 105, "A Man a Mile") He also has great sympathy for those who commit crimes of passion in an act of revenge. [citation needed]

Due to the many problems he has in his personal life, Danny's job performance often suffers; drawing many reprimands from Mac. [citation needed] This occasionally manifests itself in his appearance being less than professional on the job. Despite this, Mac knows that Danny could be a great detective if he could separate his personal from his job. [citation needed]

Danny often ignores advice, including strongly worded advice from his mentor Mac Taylor. Despite this tension, Danny's efforts are enough to keep him on Mac's team.

Danny normally has a calm and composed demeanor, though he will occasionally "go off" on a suspect, or become upset when his personal life issues surface. He also keeps a good sense of humor, often making jokes to lighten the mood. He also occasionally joins Flack in making humorous comments when talking to suspects.

Danny grew up on Staten Island, having had many brushes with the Tanglewood Boys. As a result of his brother Louie's actions on a night in 1991, the two have had a rocky relationship. Louie had been involved in a brutal beating, apparently his initiation into the Tanglewood Boys. Since that night, Danny has developed his own code of ethics, and it is clear that the experience deeply disturbed him.

When Danny was 10 he and his father got into the wrong gypsy cab and were subsequently beat; causing his hatred for gypsy cabs (episode 118, "The Dove Commission").

Danny played baseball since he was seven. He went from little league to college and even had a stint in the minor leagues before he broke his wrist (season 122, "The Closer").

After graduating from the Police Academy, Danny was snatched up by Mac Taylor, who saw in Danny a very promising detective. When Mac hired Danny, Mac was told that one day he would realize that the hiring was a mistake (episode 121, "On the Job").

Almost a year after a episode featuring a case that involved the Tanglewood Boys gang (episode 113, "Tanglewood"), Danny was implicated in a 15-year-old cold murder case. Louie was severely beaten by the gang and left in a coma after attempting to prove Danny's innocence. With Mac's help, Danny eventually cleared his name, thanks to a taped confession from the real killer (episode 220, "Run Silent, Run Deep"). Louie's condition is still unknown, though it is widely presumed that he is still comatose.

Danny takes it upon himself to trade shifts with Lindsay Monroe taking her place to assist lab tech Adam Ross at a warehouse crime scene. When he arrives, he is taken hostage and held along with Adam by Irish mobsters who hope to use the situation to distract the NYPD while others of their clan break into the lab to retrieve a huge cocaine seizure from earlier in the day. Danny is badly beaten during his captivity (taking a total of 4 blows to the head, one to the spine and having his left hand broken), but with Adam's help, manages to take down their captors in time to be rescued by the officers gathered outside (episode 324, "Snow Day").

In season four , a boy named Ruben whom Danny is friends with and also lives in his apartment building is shot accidentally during a robbery of a bodega. Ruben had been in Danny's care at the time and Danny hadn't known he was shot. Danny blamed himself for Ruben's death. Danny ends up telling Ruben's mother what happened, who later on steals his gun to find the man who robbed the bodega, who had recently gotten off on parole. Flack goes looking for Danny, who had been following Ruben's mother, Nikki, trying to get back his gun. Danny and Flack split up and find both Nikki and the man who robbed the bodega. During this time, Danny reveals just how badly he blames himself by telling Nikki it's his fault, and if she should shoot someone it should be him. Danny punches the robber of the bodega out of anger, but that isn't brought up again. Flack gives Danny the ultimatum of either turning Nikki in for stealing a cop's gun or doing it himself. Danny ends up bringing her in, but she gets off for unknown reasons. Flack tells Danny he has to stop blaming himself for Ruben's death. Danny replies with, "How?"

Having been shot during the season five finale, Danny finds himself confined to a wheelchair. He eventually recovers fully and becomes able to walk again after some physical therapy.

While receiving acupuncture, Danny's badge, wallet and dog tags are stolen from his locker. He discovers the dog tags at a local pawn shop but has no lead as to who pawned them. A finger print is found on the dog tags that leads Danny to discover Shane Casey is the one behind the theft.

In the season finale Danny, Lindsay and Lucy go on a vacation. While visiting a lighthouse, they meet Shane Casey. He tells Lindsay and Lucy to leave, because his fight is not with them. In the end, Shane falls off the lighthouse. Danny tries to help him, but Shane lets go after saying, "Don't worry, Messer. I will."

Before the episode ends, Lucy is crying, and Danny wakes up, and as he gets to her room, he finds a smiley Shane holding Lucy.Then the screen goes black and a gunshot is heard, leaving Danny's status a mystery (episode 623, "Vacation Getaway").

In season 7 premiere, it is revealed that the gunshot heard in the last episode was fired by Lindsay, who came into Lucy's room with her gun and shot Casey, killing him instantly. She was then honored for her bravery.

In the season 7 finale, "Exit Strategy", it is revealed that Danny has taken, and passed, the sergeant's exam.

As the season begins, Danny is working as a Patrol Sergeant training rookie officers. In "Officer Involved" he was framed in a IA investigation by one of his trainees who lies saying Danny told her to cover up the fact that she killed an innocent man by accident. The officer, Cooper, sticks to her story, but Lindsey is able to get her to see reason and she admits the truth before Danny would've gone before IA with Mac's backing. Danny's active status is restored, but the fact that he trusted his rookie cops and one betrayed him causes him to decide to go back to the Crime Lab with the people he knows he can trust. As a result, Danny gives up his promotion and officially returns to the Crime Lab two weeks later.

Danny: So, you wanna tell me why you won't marry me?Lindsay: I didn't mean I won't. I just think it's the wrong time.Danny: Wrong time? The wrong time, or the wrong guy?Lindsay: Danny, I know I've thrown a lot at you, and we're both feeling really overwhelmed, but whatever we do next, I want it to be for the right reason. Wouldn't you rather walk down the aisle, than be pushed?Danny: Of course.Lindsay: Okay, good, so listen. I'm not goin' anywhere, and I know you're not goin' anywhere either. Okay, but this isn't just about you and I anymore. So, let's just take baby steps, 'kay?Danny: Okay. Sure. Look, before we go inside I wanna tell you one more thing, okay? I just want you to know that I love you.Lindsay: I love you too, Danny. (episode 510"The Triangle").